Differential pressure transmitter with intrinsic verification

ABSTRACT

The present invention provides a dual sensor differential pressure transmitter with a single fill fluid volume that intrinsically eliminates process and environmental performance influences, increases signal level while substantially reducing product costs.

PRIORITY

The application takes priority from, U.S. Provisional Patent ApplicationSer. No. 61/409,631, Filed: Nov. 3, 2010, entitled: DifferentialCalibration Pressure Transmitter With Intrinsic Verification.

FIELD OF THE INVENTION

The invention relate to improved differential pressure transmitters withimproved accuracy, their methods of use and manufacture preferably forindustrial uses.

BACKGROUND OF THE INVENTION

Differential pressure transmitters require a great deal of care andmaintenance in order to function properly for their intended purposes.It is common practice for differential pressure transmitters to beremoved from the application or field installation and transported towell-equipped calibration laboratories to assure the accuracy of theirmeasurement. This practice is costly and disruptive. Furthermore,calibration laboratories rarely simultaneously duplicate the combinedactual process conditions of a specific transmitter. For example, aninadvertent over-range when re-installing the transmitter on-line willunknowingly compromise “assurance” of accuracy provided by thecalibration. Often, a compromised partial calibration is conductedwhereupon the output of the transmitter is adjusted for a zero value bya technician at the transmitter.

Unfortunately, measurement accuracy is influenced by the combination ofmany environmental process and environmental conditions such as processpressure, process temperature, environmental temperature, solarradiation, local neighboring thermal radiation, inadvertent over-range,electronic/mechanical drift and enclosure distortion. Although theseinfluences are interdependent, they are usually considered as beingindependent. Unfortunately, the user or field technician is notroutinely provided with standard techniques or methods to properlycompensate for these interdependencies and usually does not have therequired facilities.

The present practice is to compensate for these influences withoutconsidering their interdependencies. This is pragmatically achieved byerroneously applying independent corrections for the prominentinfluences. This neglect of the interdependency of the variousinfluences increases errors in measurement. Accurate compensation mustbe conducted taking into account the actual combined environmental andprocess conditions.

Conventional differential pressure transmitters having a single sensorexacerbate these detrimental influences. For example, existing singlesensor, dual fill fluid volume differential pressure transmitters whichtend to have differences in the fill fluid volumes, the spring rates andeffective area of pressure sensitive elements of the high and low sideswill produce a detrimental differential pressure due to processpressure, process temperature or enclosure distortion acting upon thesedifferences. Similarly, within single sensor, single fill fluid volumedifferential pressure transmitters having a significant difference inthe spring rate of pressure sensitive elements of the high and low sideswill produce a detrimental differential pressure due to processpressure, transmitter temperature or enclosure distortion acting uponthese differences.

These conditions impact asset management and product quality. A user,until now, has had no recourse other than to accept the poor conditions.

SUMMARY OF THE INVENTION

The many environmental and process influences referred to earlier, areexacerbated by the current differential pressure transmitters employinga single differential pressure sensor. A single sensor differentialpressure transmitter cannot compensate for process or environmentalinfluences without determining the pressure and temperature prior toapplying the compensation factors. What is proposed therefore is a noveldifferential pressure transmitter, wherein any compensation forenvironmental influences does not require monitoring of the pressure andtemperature

Specifically, the object of the present invention is to provide a dualsensor differential pressure transmitter with a single fill fluid volumethat intrinsically eliminates process and environmental performanceinfluences, increases signal level while substantially reducing productcosts.

In a first embodiment, the differential pressure transmitter of theinvention can provide improved performance at a low product cost. In asecond embodiment, the differential pressure transmitter of theinvention provides enhancements satisfying more demanding applications.In a third embodiment, the differential pressure transmitter of theinvention provides capabilities presently unavailable in the industryand satisfies the most demanding applications. All embodiments comprisea novel system and method for differential pressure-sensing andcalibration described herein.

The proposed differential pressure transmitter intrinsically eliminatesdetrimental process and environmental influences and provides a remotelyactivated assurance of the elimination of these environmental influencestraceable to NIST within +/−0.005% of the reading

With the proposed dual sensor, single fill fluid volume differentialpressure transmitter, compensation does not require monitoring of thepressure and temperature. The object of the present invention is toprovide a dual sensor with a single fill fluid volume differentialpressure transmitter that intrinsically eliminates process andenvironmental performance influences, increases signal level whilesubstantially reducing product costs.

In a first embodiment, the differential pressure transmitter comprises abody, and first and second cavities within said body connecting to afirst and a second port, respectively on the exterior of said body. Thetransmitter further comprises first and second flexible elementsassemblies within and sealed to said first and second cavities therebyforming a third and a fourth cavity and a fifth cavity connecting saidthird and said fourth cavities. The transmitter further comprises a fillfluid having a fluid fill volume within and connecting said third, saidfourth and said fifth cavities and means of sensing the first and secondposition of a first and a second flexible element end within said thirdand said fourth cavities. The transmitter further comprises means ofproviding a conditioned response from the said first and said secondposition of a first and a second flexible element end, wherein, the saidconditioned response of said first and said second flexible element endposition is proportional to the desired measurement of the saiddifferential pressure applied to said differential pressure transmitter.

In a second further embodiment the differential pressure transmitter ofthe invention, said aforementioned means of sensing the position of saidfirst and said second flexible element end is achieved by sensing thecapacitance between the said first and said second flexible element endand a first and second electrode. In one aspect said first and saidsecond electrode is located within and is electrically insulated andattached to said third and said fourth cavity. In one aspect a first anda second electrical conductor is electrically attached to said first andsaid second electrode and said first and said second electricalconductor is sealed to contain said fill fluid within said third andsaid fourth cavities and electrically insulated from said body. Anelectronic module external to said body and electrically connected tosaid first and said second conductor and said body can also be provided.The electronic module senses the capacitance between said first and saidsecond flexible element end and said first and said second electrode andprovides a conditioned response indicative of the said differentialpressure. The change in position of said first and said second flexibleelement end produces a said first and said second change in capacitancebetween said first and said second flexible element end and said firstand said second electrode and said first and said second change incapacitance is conditioned to provide a response that is proportional tothe desired measurement of said differential pressure.

In a third, further embodiment, a change in said fill fluid volume dueto temperature variation, process pressure or enclosure distortionvolume variation produces equal and opposing influences upon said firstand said second flexible element assemblies, and said first and saidsecond flexible element assemblies are produced or compensated to haveequal ratios of spring rate to effective areas, thereby causing saidtemperature variation and said process pressure variation and saidenclosure distortion to have minimal influence upon differentialpressure measurement

As mentioned above, the invention further contemplates an electronicmodule. In one embodiment, the electronic module comprises means forsensing said first capacitance between said first flexible element endand said first electrode and a said second capacitance between saidsecond flexible element end and said second electrode; means fordetermining the first and second position of said first and said secondflexible element end by sensing said first and said second capacitance;means for determining a reference zero condition position of said firstand said second flexible element end while at reference temperature andreference common pressure and no applied said differential pressure;means for determining operating zero condition position of said firstand said second flexible element end while at operating temperature andoperating common pressure and no applied said differential pressure;means for determining reference differential pressure condition positionof said first and said second flexible element end while at referencetemperature and reference common pressure and said differentialpressure; means for determining operating differential pressurecondition position of said first and said second flexible element endwhile at operating temperature, operating pressure and said differentialpressure; a means for determining a first and a second difference inoperating position between said operating differential pressurecondition position and said operating zero condition position of saidfirst and said second flexible element end; a means for providing anoutput proportional to said first and a second difference in operatingposition between said operating differential pressure condition positionand said operating zero condition position of said first and said secondflexible element end; a means for determining a first and a seconddifference in reference position between said reference differentialpressure condition position and said reference zero condition positionof said first and said second flexible element end; and a means forproviding an output proportional to the said first and a seconddifference in reference position between said reference differentialpressure position and said reference zero position of said first andsaid second flexible element end.

In another embodiment, the electronic module of the invention comprisesmeans for determining said fill fluid temperature; means for determiningsaid fill fluid pressure; means for calculating the change in saidoperating differential pressure condition from reference zero conditiondue to a change in said fill fluid temperature; means for calculatingthe change in said operating differential pressure condition from saidreference zero condition due to a change in said fill fluid pressure;means for providing an output of said temperature; means for providingan output of said pressure; and means for providing an output of saidreference zero condition thereby determining a reference zero condition.

The differential pressure transmitter of the invention may optionallyfurther comprise a three-position valve. In one embodiment the threeposition valve comprises: a valve body having a first external port anda second external port to external pressures and said body having twointernal transmitter ports a first internal port and a second internalport connecting to said differential pressure transmitter; a rotaryvalve plug having two internal flow conduits; and means of positioningsaid rotary valve plug to any of three-positions. In one aspect, thethree positions of the valve are as follows: a first position whereinthe first external port is connected to the first internal port and thesecond external port is connected to the second internal port; a secondposition wherein the first internal port is connected to the secondinternal port and no connection made between the first and secondexternal ports external ports; and a third position wherein the firstexternal port is connected to the second internal port and the secondexternal port is connected to the first internal port. In this aspect,normal operation of said differential pressure transmitter is configuredper said first position, process isolation and said differentialpressure transmitter equalization is configured per said second positionand reverse operation of said differential pressure transmitter isconfigured per said third and wherein prior to entering said first orthird positions said three-position valve enters said second position

In another aspect, the aforementioned three position valve may furthercomprise means of determining said reference zero position by isolationof said differential pressure transmitter from said process whilemaintaining process pressure upon said differential pressure transmitterand equalization of the said differential pressure upon saiddifferential pressure transmitter in said second position said threeposition valve and whereby, without any said differential pressure orwith constant said differential pressure, the said differential pressuretransmitter output in said normal operation is compared to the saiddifferential pressure transmitter output in said reverse operation andprovides an indication of the differences in density and/or liquidheight of process fluid in the impulse lines connected to saiddifferential pressure transmitter and thereby provides a means for thecompensation of impulse line density and level influences.

In one aspect the abovementioned electronic module implements a methodfor compensating the combined influence of said temperature and saidpressure due to said change in said fill fluid fill volume, said springrates and said effective areas of said first and said second flexibleelement assemblies. In one embodiment the method comprises isolatingsaid differential pressure transmitter from said process whilemaintaining said process pressure and said temperature within saiddifferential pressure transmitter and allowing equalization of said highside and said low side; sensing said process pressure with a processpressure sensor; sensing said temperature with a process temperaturesensor; sensing said operating zero condition of said first and saidsecond flexible element at said process pressure and said temperature;calculating the deflection of said first and second flexible element dueto said process pressure; calculating the deflection of said first andsecond flexible elements due to said process temperature; alculating theratio of said spring rate to said effective area of said first andsecond flexible elements; calculating the said spring rates of saidfirst and second flexible elements; calculating the said areas of saidfirst and second said flexible elements; and generating and applyingcompensation factors for said first and said second flexible elementsfor said process pressure and said temperature. The aforementionedmethod compensating for the said differential pressure transmitter forinfluences of the combined influence of said temperature and saidpressure due to said change in said fill fluid fill volume, saideffective areas and said spring rates of said first and said secondflexible element assemblies.

In another embodiment the differential pressure transmitter of theinvention may optionally comprising a three position actuator. In oneembodiment the three-position actuator comprises a first cylinder havinga first piston and a first pressure port, the first cylinder having astop for limiting axial motion of said first piston; and a secondcylinder having a second piston, said second cylinder having an axialslot and said second piston having a radial extension positioned withinsaid axial slot of said second cylinder; and a third cylinder having athird piston and a second pressure port the third cylinder furthercomprising a stop for limiting axial motion of said third piston. Afirst actuator position is obtained by pressure being applied to saidfirst cylinder through said first port, a second actuator position isobtained by pressure being applied to said third cylinder through saidsecond port and a third position is obtained by pressure applied to saidfirst cylinder through said first port and to third cylinder though saidsecond port. Positioning of said center piston moves said three-positionvalve to said position one, said position two or said position three andsaid radial extension of said second piston provides a means of movingan external device to any of the said three positions.

In another embodiment, the transmitter may optionally include agravitational pressure reference source. In one aspect, thegravitational pressure reference source comprises: a body; an internalcavity having a post; a sphere having a hole containing termination ofsaid post that is attached to said sphere is sealed to said post; acylinder having enlarged internal diameters at each end; a steppedcylindrical post attached to said cylinder; a cylindrical weight with aninternal diameter accepting said stepped cylindrical post; and a meansof securing said stepped cylindrical post to said cylindrical weight,wherein said cylinder, said stepped cylindrical post and saidcylindrical weight comprise a gravitational reference assembly. Thegravitational pressure reference source further comprises an internalcylindrical magnet within a cavity in said body and vertically below andconcentric with said gravitational reference assembly wherein the saidinternal cylindrical magnet can be raised by an external magnet fieldwith opposing magnetic poling and said raising of said internal magnetraises said gravitational reference assembly relative to said sphere andwherein upon a change of said external magnet field the said internalcylindrical magnet falls rapidly due to gravity and the said change insaid external magnet field and wherein the gravitational referenceassembly falls under the action of gravity producing a referencepressure in the said cavity of the said cylinder and wherein saidreference pressure is applied to the internal cavity of the post. Thegravitational pressure reference source may also include a means ofmeasuring temperature by capturing the time of the descent for a knowndistance of the said gravitational reference assembly and a means forconverting the said time of a descent for a said known distance to anaverage velocity of the said fill fluid through said gravitationalreference assembly and from said average velocity through saidgravitational reference assembly determine a viscosity of said fillfluid and from said viscosity determine said temperature from knownviscosity versus temperature relationships. The response of thedifferential pressure transmitter upon the application of the gravitypressure reference provides a means of sensing said reference pressurefor verifying calibration and determining said temperature of said fillfluid.

The differential pressure transmitter may also further comprise anactuator for actuating said gravitational pressure reference. In oneembodiment the gravitational pressure reference actuator comprises: (a)a piston having a longitudinal axis, said piston having four cavitieswith an axis of symmetry perpendicular to and intersecting saidlongitudinal axis of said piston and said axis of symmetry of said fourcavities and said longitudinal axis are parallel and said piston havingfour magnets contained within the said cavities and the magnetic polingof each said magnet alternates along said piston longitudinal axis; and(b) a cylinder with a first and a second closed end wherein said pistonand said magnets are contained within said cylinder and said piston andsaid cylinder having means for preventing rotation of said piston withinsaid cylinder. The cylinder has a first and a second pneumatic portlocated at a first and second closed end of said cylinder respectively.By applying pneumatic pressure to the first pneumatic port the piston ismoved to the second closed end. Likewise by applying pneumatic pressureto the second pneumatic port the piston is moved to the first closed endof the cylinder.

In one aspect, the magnets of the aforementioned gravitational pressurereference actuator within said process enclosure are raised by externalmagnets by providing an axial opposing magnetic field. Likewise, saidmagnets within a said process enclosure are lowered by said externalmagnets by providing an axial additive magnetic field and wherein meansis provided for actuating said gravitational pressure reference.

In one embodiment the flexible element assemblies of the differentialpressure transmitter of the invention may comprise one or more axialthin cylindrical sections having, at each said axial end, a thin radialextension and wherein said thin radial extensions of successive saidaxial thin cylindrical sections are joined at the outermost radialposition and wherein one of said axial thin cylindrical sections havingsaid thin radial sections at said axial end is joined to a supportmember and opposing said axial thin cylindrical section of the said oneor more axial thin cylindrical sections having said thin radial sectionat axial end is joined to an end member and said radial sections arenormally distended in the said axial direction. Upon application of ahigh value of said external process pressure the said thin radialsections deflect axially until restrained by said support member and bymating of said radial sections and said flexible element ends arecapable of returning to original condition after enduring said externalprocess pressure due to the low stress encountered

In one aspect of the invention the transmitter of the inventioncalculates a correction factor as follows. An external, equal and commonpressure is applied to the said first and second flexible elementassemblies The deflection of each of said flexible element assemblies asa result of the said compression of said fill fluid due to said pressureis sensed. The difference in the ratio of spring rate to effective areaof a said first and second flexible element assemblies is determined bycomparing the said displacements of the said pair of flexible elementends in response to the said common pressure. A correction factorconsisting of the ratio of spring rate to effective areas of said firstand second flexible element assemblies is produced and is used tocompensate for said deflections of said first and second flexibleelement assemblies in the sensing of said differential pressures.

In addition to process and environmental influences, over-range of thedifferential pressure transmitter is a major influence and usually notspecified or considered. If specified, it usually does not apply toworst-case conditions resulting from a combination of maximum workingpressure while at maximum process temperature. The proposed differentialpressure-sensing concept minimizes these over-range concerns due tohysteresis from over stressing by an assurance that the proposed conceptis not highly stressed and well supported during the over-range. Zeroand span return errors from overstressing as in present practice aresignificantly minimized. Thus an improvement in over-range performanceis inherent in the proposed differential pressure-sensing concept andresolves the worst-case condition of maximum process pressure over-rangeat maximum process temperature.

The proposed dual sensor, single fill fluid volume differential pressuretransmitter is shown in FIG. 1 and the dual sensor concept is shown incross-section in FIG. 2. This proposed dual sensor concept does noteliminate the undesirable change in fill fluid volume occurring withchanges in pressure, temperature or enclosure distortion but it doesintrinsically eliminate the undesirable error influence. Anydifferential pressure developed due to the change in fill fluid volumefor whatever cause is applied equally and opposingly to the high and thelow side flexible element assemblies with no differential pressure beingsensed by the differential pressure transmitter. Ideally, if thecombined response of spring rates and effective areas of the high andlow side flexible element assemblies are matched, there cannot be adifferential pressure developed in the proposed concept due to thedetrimental influences.

Optimization of the proposed concept requires design and manufacturingconsiderations to assure this match of the combined response of springrates and effective areas of the high and low side flexible elementassemblies of (3A) and (3B) of FIG. 2. Although these efforts mayproduce a good match, it cannot be assured to be insignificant. However,an innovative simple manufacturing procedure assures these differencesin the spring rates and effective areas of the high and low sideflexible element assemblies due to manufacturing tolerances areinsignificant. During the manufacturing process, a high pressure issimultaneously applied to the high and low side flexible elementassemblies while monitoring the deflections of the high and low sideflexible element ends resulting from the compression of the fill fluid.The difference in the deflection of the flexible element ends provides ameans of compensating for the difference in the effective areas andspring rates of the flexible element assemblies. The equation forcompensation will be developed further in the discussion and illustrateshow the compensation is implemented. Thusly, the difference in thespring rates and effective areas of the flexible element assemblies dueto manufacturing tolerances is minimized and ideally eliminated assuringa high level of performance. Furthermore, this process can also beapplied in the field. Thus, a user can verify high performance uponreceipt and during routine maintenance.

The proposed dual sensor, single fill fluid volume differential pressuretransmitter is simple in construction. A single fill fluid chamberexists between the high side flexible element assembly and low sideflexible element assembly. Within this single chamber, there are fixedelectrodes (4 a) and (4 b) of FIG. 2 that are in close proximity to eachof the flexible element ends (8A) and (8B). The sensing is achieved bysimultaneously measuring the differential change in capacitance due tothe deflection of the flexible element end with respect to the fixedelectrode for the high and the low side. A pressure applied to the highside deflects the flexible element end of the high side inwardly towardsthe fixed electrode and simultaneously the fill fluid causes the lowside flexible element end to deflect outwardly away from the fixedelectrode due to the equal displaced volume of the flexible elementsassemblies.

Operation of the differential pressure transmitter in a flow or levelapplication, is categorized by four conditions that may be defined:

1. When the transmitter is assured to be at a reference temperature,reference process pressure and no differential pressure, the output isdefined as “reference zero condition”.

2. When the transmitter is assured to be at a known temperature, knownprocess pressure and no differential the output is defined as “operatingzero condition”.

3. When the transmitter is assured to be at a known temperature, knownprocess pressure and a known differential pressure with respect to“reference zero condition” is defined as “reference differentialpressure condition”.

4. When the transmitter is assured to be at a known temperature, knownprocess pressure and at a differential pressure being measured, it isdefined as “operating differential pressure condition”.

The proposed advanced and premium differential pressure transmitterconcepts will satisfy the requirements of more demanding applications.The advanced and premium differential pressure transmitters are composedof the standard differential pressure transmitter with ancillarydevices.

There are three ancillary devices:

The advanced and premium product has an actuator that remotely operatesa three-position valve for normal, equilibrate or reverse position. Theequilibrate position isolates the transmitter from the process.

The premium product also incorporates a gravitational pressure referencethat verifies calibration traceable to National Institute of Standardswith an actuation device that provides remote operation of thegravitational pressure reference.

The proposed premium dual sensor, single fill fluid volume differentialpressure transmitter concept provides capabilities that presently arenot available in the industry and will now be described.

Differential pressure transmitters have been improved in recent years.An example is provided in U.S. Pat. No. 6,321,585 Sgourakes for aDifferential Pressure Generator. This improvement eliminates alldetrimental combined interdependent process and environmental influencesof differential pressure transmitters by remotely verifying measurementaccuracy within +/−0.005% of reading traceable to National Institute ofStandards while transmitter is on-line at process and environmentaloperating conditions for flow and liquid level applications.

The present invention integrates U.S. Pat. No. 6,321,585 SgourakesDifferential Pressure Generator within the proposed premium differentialpressure transmitter and with the addition of proposed ancillarydevices, provides an exceptional high-performance premium differentialpressure transmitter for flow and liquid level applications with remotecalibration assurance.

The premium differential pressure transmitter provides significantadvancements in performance. Some of the advancements enhancing theremote calibration verification of U.S. Pat. No. 6,321,585 Sgourakes fora Differential Pressure Generator are:

-   -   1. A reference zero condition value is available with each        differential pressure observation providing an ability to        monitor zero conditions during normal differential pressure        measurement.    -   2. Detrimental influences of environmental temperature, process        temperature and process pressure are intrinsically eliminated        from the differential pressure transmitter.    -   3. Automatically scheduled sensor calibrations can be achieved        remotely during routine sustained operation.    -   4. Reverse flow capability. The three-position valve provides an        ability to measure normal or reverse flows.    -   5. Elimination of density or level differences in impulse lines        is assured. This is achieved by comparing the zero condition in        normal and reverse positions of the three-position valve. Any        difference can be attributed to density or level differences in        the impulse lines and the influence compensated.    -   6. The transmitter provides greater range limits by providing        lower span capability avoiding the cost and complexity of        multiple transmitters with intermediate spans.    -   7. Minimal over-range influence.    -   8. A very low cost of manufacture.    -   9. Calibration is assured to be within +/−0.005% of reading        traceable to National Standards Institute, achieved from remote        locations without a technician present at transmitter, at actual        combined operating conditions, on-line and without flow        interrupt.    -   10. Pro-active maintenance can respond if a trend of concern        develops from sequential calibration assurances or from        monitoring of the zero value at each differential pressure        acquisition.    -   11. Instantaneous assurance of proper operation can be remotely        verified within minutes during crisis conditions.    -   12. Eliminates the need to interrupt signal, remove transmitter        from process line, hand written manual calibration history        management, lengthy evaluations in a calibration laboratory        requiring the simulation of process pressures and environmental        temperatures.    -   13. Provides a capability for remotely scheduled customer/buyer        audits eliminating skilled operators, costly travel, hotel        accommodations and seasoned resources.    -   14. The present capacitive single sensor concepts are typically        a stretched diaphragm with an effective area of approximately ⅓        inch squared with non-linear deflection. Conversely, the        proposed capacitive concept has an effective area of 3.5 inches        squared with linear deflection. Thus providing a factor of ten        improved sensitivity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view of the differential pressure transmitter.

FIG. 2 is a cross sectional view of the proposed differential pressuresensor of the differential pressure transmitter.

FIG. 3 is an isometric view of the premium differential pressuretransmitter with integrated three-position valve and valve operator andgravitational reference with operator.

FIG. 4 is a schematic view illustrating the three position hydraulicconnections.

FIG. 5 is a view showing the three-position valve components in thenormal position.

FIG. 6 is an isometric view of the three position valve components inthe equilibrate position.

FIG. 6A is a cross sectional view of the three position valve in theequilibrate position with center piston positioned in center position.

FIG. 7 is a cross sectional view of the gravitational pressure referencewith the actuator in the normal run position.

FIG. 8 is a cross sectional view of the gravitational pressure referencewith the actuator having raised the weight and cylinder assembly andprepared to initiate development of the gravitational pressurereference.

DETAILED DESCRIPTION OF THE INVENTION

The proposed dual sensor, single fill fluid volume differential pressuretransmitter (1) is illustrated in FIG. 1 with the major components shownas a body (2), two process interface assemblies (3A) and (3B), highpressure process port (12) and low pressure process port (13).

The dual sensor, single fill fluid volume differential pressuretransmitter (1) of FIG. 1. is very compact and optimized to accommodatepresent impulse line spacing of 2⅛″ between high-pressure process port(12) and low-pressure process port (13). The flexible element assembly(3A) of FIG. 2, is composed of a flexible element end (8A) and twoconvolutions (9AA) and (9AB). The flexible element assembly (3A) isattached to a base (15A) having an isolation groove (16A) that minimizesinfluences from distortion of the body (2) due to process pressure orprocess/environmental temperature. Additional components are the fillfluid (14), fill fluid connecting tube (11) and fill fluid filling ports(10A) and (10B).

The dual sensor measures the differential pressure by sensing thecapacitance change due to the deflection of flexible element end (8A)with respect to the fixed electrode (4A) as shown in cross section 2—2of FIG. 2. and simultaneously the deflection of flexible element end(8B) with respect to the fixed electrode (4B). The flexible elementassemblies (3A) and (3B) thereby provide process isolation and adifferential pressure sensing capability.

The flexible element assembly (3A) has an electrode (4A) mounted upon aninsulator (5A) that is attached to the base (15A). The electrode (4A)has an electrical conductor (6A) providing electrical continuity fromthe electrode (4A) to an electrical termination (17A) of hermetic seal(7A). The electrical conductor (6A) has a stress relief (not shown) thatminimizes thermal expansion and pressure expansion influences to assurereliable connectivity between electrode (4A) and the electricaltermination (17A) of the hermetic seal (7A). Additionally, theelectrical conductor (6A) is contained within an insulator (18A) tominimize undesirable capacitive coupling and restrict relative motionbetween the conductor (6A) and the body (2).

A fill fluid (14) hydraulically couples the flexible element assembly(3A) of the high side to the flexible element assembly (3B) of the lowside. Thus a high pressure applied to a flexible element assembly (3A)of the high side causes an inward deflection while the opposing flexibleelement assembly (3B) experiences an outward deflection.

The equations predicting the differential pressure considering theposition of the flexible element ends (8A) and (8B) and the ratio ofspring rate to effective area of the flexible elements (9AA), (9AB),(9BA), and (9BB) of FIG. 2. are developed as follows:

Definitions:

-   PHS=Pressure sensed on high side-   PI=Internal pressure of fill fluid-   P=Process pressure on high and low side-   KH=Spring rate flexible element assembly high side-   KL=Spring rate flexible element assembly low side-   AH=Effective area flexible element assembly high side-   AL=Effective area flexible element assembly low side-   DHR=Position of high side flexible element end with PHS-   DHZ=Position of high side flexible element end without differential    pressure-   DLR=Position of low side flexible element end with PHS applied to    high side-   DLZ=Position of low side flexible element end with no differential    pressure

The summation of the forces applied to flexible element ends aredetermined as follows:AH*(PHS+P−PI)−KH*(DHR−DHZ)=0 Sum of forces on high side flexible element

${{PHS} + P - {PI}} = \frac{{KH}*\left( {{DHR} - {DHZ}} \right)}{AH}$d/p of high side flexible elementAL*(PI−P)−KL*(DLR−DLZ)=0 Sum of forces on low side flexible element

${{PI} - P} = \frac{{KL}*\left( {{DLR} - {DLZ}} \right)}{AL}$d/p of low side flexible element

${PHS} = {{\frac{KH}{AH}*\left( {{DHR} - {DHZ}} \right)} + {\frac{KL}{AL}\left( {{DLR} - {DLZ}} \right)}}$Desired equation for differential pressure

Thus the sum of the deflections of the flexible element ends isproportional to the differential pressure.

This equation requires the actual value of each ratio of spring rate toeffective areas of the flexible element assemblies be known.Alternatively, an innovative procedure has been developed. In thisprocedure, a high process pressure is applied to the high processpressure port (12) and simultaneously to the low process pressure port(13) thereby compressing the fill fluid volume (14). The compression ofthe fill fluid is sensed by the deflection of each flexible element end.The ratio of these deflections provides a means of compensating theratios of spring rate to effective area of the two flexible elementassemblies. The compensation is developed as follows:

Definitions:

PI=Process pressure internal

P=Process pressure high and low side

DLP=Position of low side DHP=Position of high side

T=Temperature difference from a reference temperature

a=Coefficient of thermal change in volume

b=Bulk Modulus Coefficient of pressure change to volume change

A force balance summation of each flexible element assembly provides thedesired relation to be used in the compensation.

(P − PI) * AH − KH * DHP = 0 (P − PI) * AL − KL * DLP = 0${P - {PI}} = \frac{{KH}*{DHP}}{AH}$${P - {PI}} = \frac{{KL}*{DLP}}{AL}$

${DHP} = \frac{DLP}{\frac{{KH}*{AL}}{{AH}*{KL}}}$Equating pressures and solving for desired ratio

$K = \frac{{KH}*{AL}}{{AH}*{KL}}$ ${DHP} = \frac{DLP}{K}$Abbreviate

The ratios of spring rate to effective area of the two flexible elementassemblies can now be compensated using this factor. Compensation isachieved by arbitrarily selecting the high side flexible elementassembly as a reference and applying the compensation factor to the lowside flexible element assembly. Thus the compensated equation becomes:

${PHS} = {{\frac{KH}{AH}*\left( {{DHR} - {DHZ}} \right)} + {\frac{KH}{AH}*\frac{{DLR} - {DLZ}}{K}}}$

The compensation also requires a change in reference from KL/AL to KH/AHfor the low side.

Thusly, the desired differential pressure can be sensed from thedeflection of the compensated flexible element assemblies without a needto determine the actual value of the spring rate or effective area ofeach flexible element assembly. An overall calibration coefficient wouldinclude the ratio of spring rate to effective and an additional factorfor setting the output for a given input.

It will now be shown how the compensated equation intrinsicallyeliminates the detrimental influences of process and environmentalinfluences. A change in the common fill fluid volume will cause an equaland opposing change in the differential pressure applied upon each ofthe flexible element assemblies but will not cause any change in thetotal differential pressure sensing. This is an important and basicbenefit, for process temperature, process pressure, environmentaltemperature and enclosure distortion will change the common fill fluidvolume. Therefore the detrimental performance influences areintrinsically eliminated.

An equation considering the detrimental influences will illustrate themanner in which they are intrinsically eliminated. The deflectionassociated with the detrimental differential pressure due to anincreased process pressure compressing the fill fluid volume can bedetermined from the following equations:

${DPH} = \frac{P*\beta*V}{2*{AH}}$${DPL} = {\frac{P*\beta*V}{2*{AL}}*\frac{AL}{AH}}$ DPH = DPL

Similarly, The deflection associated with the detrimental differentialpressure due to due to an increased temperature expanding the fill fluidvolume can be determined from the following equations:

${DTH} = \frac{T*\alpha*V}{2*{AH}}$${DTL} = {\frac{T*\alpha*V}{2*{AL}}*\frac{AL}{AH}}$ DTL = DTHIncluding these influences within the basic equation provides:

${PHS} = {{\frac{KH}{AH}*\left( {{DHR} - {DHZ}} \right)} + {DPH} - {DTH} + {\frac{KH}{AH}*\frac{{DLR} - {DLZ}}{K}} - {DPL} + {DTL}}$

This complete, compensated equation reveals that the detrimentalinfluences are equal and opposing and are therefore intrinsicallyeliminated. The need to continually sense the process pressure andprocess temperature and apply an instantaneous compensation iseliminated.

AH and AL can be verified with the three-position valve in theequilibrate position. With the addition of a temperature and pressuresensors, an awareness of the thermal coefficient of volumetric changeand the bulk modulus of the fill fluid, the fill fluid volume and thesensed total deflection DTPH and DTPL provides a means to determine AHand AL.

DTPH = DTH + DPH  and  DTPL = DTL + DPL $\begin{matrix}{{DTH} = \frac{T*\alpha*V}{2*{AH}}} & {{DPH} = \frac{P*\beta*V}{2*{AH}}}\end{matrix}$${AH} = {\frac{V}{2*{DTPH}}*\left( {T*\alpha*P*\beta} \right)}$Similarly:

${AL} = {\frac{V}{2*{DTPL}}*\left( {{T*\alpha} + {P*\beta}} \right)}$

Ancillary Devices

The ancillary devices providing the desired enhancements of thedifferential pressure transmitter (1) are the three-position valve,valve actuator, gravity pressure reference and the gravity referenceactuator. All ancillary devices are contained within an assembly (14) ofFIG. 3. They will be described sequentially in the followingdescription.

The three-position valve configures the proposed differential pressuretransmitter (1) for normal, equilibrated or reverse operation and areshown schematically in FIG. 4. The main components of the proposedthree-position valve and valve operator (20) are shown in FIG. 5 and nowconsidered.

The normal position of FIG. 4. connects a high-pressure process port toa high-pressure differential pressure transmitter port and alow-pressure port to a high-pressure differential pressure transmitterwith a normal flow direction.

Equilibrate position of FIG. 4. connects a high-pressure differentialpressure transmitter port to a low-pressure differential pressuretransmitter port equilibrating pressures and no differential pressurebeing applied to the differential pressure transmitter.

Reverse position of FIG. 4. connects a high-pressure process port to ahigh-pressure differential pressure transmitter port and a low-pressureprocess port to a low-pressure differential pressure transmitter portproviding reverse flow measurement capability. Although the differentialpressure transmitter (1) remains in the same position, the high-pressureand low-pressure ports of the reverse position of the differentialpressure transmitter (1) are opposite the high-pressure and low-pressureports of the normal position.

The three position valve and operator (20) as shown in FIG. 5 iscomposed of a fixed valve seat (21) that is restricted from rotation bya matching keyway in the body (2) that is not shown and provides theports for communication with the differential pressure transmitter (1),a selector disc (22) that is rotated to configure the desired positionsof FIG. 4, a compensation plate that is not shown, provides axialcompensation for thermal and pressure deflections and torsionallycouples selector disc (22) to rotor (24), an axial spring (23) thatprovides a load to selector disc (22) and rotor (24) assuring thatselector disc (22) achieves a seal with valve seat (21) whilecompensating for thermal and pressure deflections, rotor (24) is drivenby a crank (26) of three position actuator.

The novel three-position actuator of the three-position valve (20) isshown in cross section 2-2 of FIG. 6A) for the equilibrate position. Thecenter piston (29) is driven to the equilibrate position by applyingpressure to port (33) that acts upon piston (30) forcing it to the rightuntil arrested by stop (35) in cylinder of lower molding (32) andsimultaneously applying pressure to port (34) that acts upon piston (31)forcing it to the left until arrested by stop (36) in the cylinder oflower molding (32).

The normal and reverse positions of the valve actuator are achieved bymotion of three pistons (30), (31) and (32) having an innovativesequence. Referring to FIG. 6A, when the pneumatic port (33) on the leftis pressurized, the left piston (30) travels to the right and engagesthe center piston (29) and sequentially engages the right piston (31)and continues to the right until piston (30) is limited by a stop (35)at this time the pressure is applied to center piston (29) through path(38A) and piston (31) is then driven to the right termination of thecylinder. Similarly, when the pneumatic port (34) on the right ispressurized, the right piston (31) travels to the left and engages thecenter piston (29) and sequentially engages the left piston (30) andcontinues to the left until piston (31) is limited by a stop (36) atthis time the pressure is applied to center piston (29) through path(38B) and piston (30) is then driven to the left termination of thecylinder.

Motion of piston (29) of FIG. 6A actuates the valve. A post (37) of thecenter piston (29) is attached to valve plate (28) and valve plate (28)is coupled to a crank (26). As post (37) is positioned to the left,center and the right, it rotates the crank (30) of the three-positionvalve (20). The crank (26) turns the rotor (24) that positions theselector disk (22) to the desired valve position. The valve may also beoperated manually by positioning valve plate (28) by hand. Valve plate(28) provides an indication of the position of the valve.

The three-position valve (20) provides the ability to determine andremove the influence of level or density in impulse lines. With aconstant flow or ideally no flow, the three position valve (20) is firstpositioned in the normal position and the normal value of thedifferential pressure transmitter (1) is determined. Then thethree-position valve (20) is positioned in the reverse position and thereverse value of the differential pressure transmitter (1) isdetermined. The results are compared and a correction made to minimizeany level or density differences in the impulse lines.

The gravity pressure reference (40) shown in cross section 3—3 of FIG.7, functions is described in detail in U.S. Pat. No. 6,321,585 Sgourakesfor a Differential Pressure Generator. However, the basic operation isas follows:

The weight and cylinder assemblies (43A) and (43B) are raised withrespect to fixed spherical pistons (41A) and (41B) and then allowed todescend under the action of gravity thereby producing a traceable,reliable reference pressure within the cylinders (42A) and (42B) that isapplied to the differential pressure transmitter (1).

The principle of operation is simple. The weight and cylinder assembly(43A) on the high side has the same volume as the weight and cylinderassembly (43B) on the low side. The desired reference differentialpressure is developed by a density difference of the weight and cylinderassembly (43A) with respect to the weight and cylinder assembly (43B).The density of the fill fluid changes significantly due to volumechanges with respect to pressure or temperature. However, the fill fluidchanges produce equal influences upon the assemblies and therefore donot influence the desired reference differential pressure. Thus thereference differential pressure is not influenced by fill fluid densityvariations that occur with temperature or process pressure.

Innovative concepts have now been provided to enhance the raising andthe descent of the weight and cylinder assemblies (43A) and (43B) ofFIG. 7. Located within the enclosure are internal magnets (45A) and(45B) that are raised by an opposing magnet field or lowered by anattractive magnetic field. These magnet fields are produced externally.

Positioning an external magnet (48) having an opposing magneticorientation to the internal magnet (45) produces an opposing magneticfield that raises the internal magnet. Positioning an external magnet(48) having an attractive magnetic orientation to the internal magnet(45) produces an attractive magnetic field that lowers the internalmagnet.

The positioning of the external magnets with respect to the internalmagnets is simply done by shuttling the external magnets horizontallyleft or right a distance equal to the one half the horizontal distancebetween the internal magnets (45A) and (45B). This motion is illustratedin FIG. 7 illustrating the relationship in normal operation desiring tocapture the internal magnets by providing an attractive field and reducevibration of the internal magnets. Fewer magnets could be used but thedesired advantage of capturing the internal magnets in normal operationthereby reducing pressure pulsations would not be achieved.

In the moment prior to the descent of the weight and cylinder assemblies(43A) and (43B) the internal magnets are held in a position illustratedin FIG. 8. To initiate a descent the external magnets (48) are quicklyreturned to the normal position. At this time the weight assemblies(43A) and (43B) experience a gravitational force that is applied uponthe effective area defined by the sphere within the cylinder therebyproducing the desired differential pressure.

The positioning of the external magnets is achieved by pneumaticpressure applied to either end of the piston (47) carrying the externalmagnets (48)

What is claimed is:
 1. A differential pressure transmitter, comprising:a pair of flexible element assemblies each of which is in fluidcommunication with a respective port, each of said flexible elementassemblies characterized by a ratio of a spring rate to effective area,each of said flexible element assemblies comprising: one or moreconvolutions, each of said convolutions comprising at least one axialcylindrical section and at least a radial extension extending from saidcylindrical section, a flat element end that exhibits a lineardeflection in response to a pressure differential applied thereto, andan electrode capacitively coupled to said element end for sensing adeflection thereof in response to said applied pressure, a connectortube containing a fill fluid for hydraulically coupling said flexibleelement assemblies, and means for determining a differential pressureapplied to said ports based on the sensed deflections of said elementends of said pair of flexible element assemblies and for compensatingfor a difference, if any, between the ratios of spring rate to effectivearea of said flexible element assemblies.
 2. The differential pressuretransmitter of claim 1, wherein each of the electrodes of said flexibleelement assemblies is mounted on an insulator attached to a base.
 3. Thedifferential pressure transmitter of claim 2, wherein said basecomprises one or more isolation grooves.
 4. The differential pressuretransmitter of claim 1, wherein each of the electrodes comprises anelectrical conductor.
 5. The differential pressure transmitter of claim4, wherein said electrical conductor of each of said electrodes extendsto an electrical termination.
 6. The differential pressure transmitterof claim 1, wherein each of said flexible element assemblies is disposedwithin a cavity sealed from the external environment.
 7. Thedifferential pressure transmitter of claim 1, wherein said means fordetermining the differential pressure utilize the ratio of spring rateto effective area of one of the flexible element assemblies as areference and applies a compensation factor to a signal received fromthe other flexible element assembly so as to simulate a condition inwhich said flexible element assemblies exhibit the same ratio of springrate to effective area.
 8. The differential pressure transmitter ofclaim 7, wherein said flexible element assemblies have equal ratios ofspring rate to effective area.
 9. The differential pressure transmitterof claim 1, wherein said flexible element assemblies are produced orcompensated to exhibit equal ratios of spring rate to effective area.10. The differential pressure transmitter of claim 1, wherein said meansfor determining the differential pressure comprises an electronic moduleexternal to said body.
 11. The differential pressure transmitter ofclaim 10, wherein said electronic module comprises: means for sensing afirst capacitance value between the flat element end and the electrodeof one of said assemblies and a second capacitance value between theflat element end and the electrode of the other assembly, means fordetermining deflections of the flat element ends of the two assembliesrelative to reference positions based on said first and secondcapacitance values, wherein the differential pressure is proportional toa difference of the deflections of the flat element ends.
 12. Thedifferential pressure transmitter of claim 1, further comprising athree-position valve, said valve comprising: a valve body having firstand second internal ports each of which is connected to one of saidports that are in communication with said flexible element assemblies,and first and second external ports for coupling to external pressuresources, a rotary valve plug having two internal flow conduits, andmeans for positioning the rotary valve plug to any of three positions,wherein in a first position the first external port is connected to thefirst internal port and the second external port is connected to thesecond internal port, in a second position the first internal port isconnected to the second internal port and no connection exists betweenthe first and second external ports, and in a third position the firstexternal port is connected to the second internal port and the secondexternal port is connected to the first internal port.
 13. Thedifferential pressure transmitter of claim 1, further comprising agravitational pressure reference source, said gravitational pressurereference source comprising: first and second weight and cylinderassemblies disposed in vertical enclosure and configured to move up anddown relative to first and second spherical pistons to apply referencepressures to said flexible element assemblies, wherein the weight andcylinder assemblies include weights of equal volumes and differentdensities, first and second internal magnets disposed in said enclosurebelow said first and second weight and cylinder assemblies,respectively, three or more magnets external to said enclosure, saidexternal magnets being movable relative to said internal magnets betweena first position and a second position, wherein in said first position,two of said external magnets magnetically repulse said first and secondinternal magnets, respectively, so as to effect raising of said firstand second weight and cylinder assemblies, and wherein in said secondposition, two of said external magnets magnetically attract said firstand second internal magnets, respectively, so as to initiate a descentof said first and second weight and cylinder assemblies under influenceof gravity and capture said first and second internal magnets oncompletion of descent.
 14. A differential pressure transmitter,comprising: a pair of flexible element assemblies each of which is influid communication with a respective port, each of said flexibleelement assemblies characterized by a ratio of a spring rate toeffective area, a connector tube containing a fill fluid forhydraulically coupling said flexible element assemblies, and anelectronic module for determining a differential pressure applied tosaid ports and for compensating for a difference, if any, between theratios of spring rate to effective area of said flexible elementassemblies.
 15. The differential pressure transmitter of claim 14,wherein each of said flexible element assemblies comprises: a flatelement end, and an electrode capacitively coupled to said flat elementend for sensing a deflection thereof in response to said differentialpressure.
 16. The differential pressure transmitter of claim 15, whereinsaid electronic module senses a change in capacitance between the flatelement end and the respective electrode of each of said assemblies. 17.A differential pressure transmitter, comprising: a body, first andsecond cavities formed within said body, said first and second cavitiesbeing connected, respectively, to first and second ports, first andsecond flexible element assemblies disposed within said first and secondcavities, each of said flexible element assemblies characterized by aratio of a spring rate to effective area responsive to an appliedpressure differential, each of said flexible element assembliescomprising: one or more convolutions, each of said convolutionscomprising at least one axial cylindrical section and at least a radialextension extending from said cylindrical section, a flat element endthat exhibits a linear deflection in response to a pressure differentialapplied thereto, and an electrode capacitively coupled to said flatelement end for sensing a deflection thereof in response to said appliedpressure, a connector tube containing a fill fluid for hydraulicallycoupling said flexible element assemblies, and an electronic moduleexternal to said body for determining a differential pressure applied tosaid ports based on the sensed deflections of said element ends of saidpair of flexible element assemblies and for compensating for adifference, if any, between the ratios of spring rate to effective areaof said flexible element assemblies.